Communication devices serve for recording or reproducing a message and for transmitting or receiving a message signal derived therefrom. The messages are generally in the form of voice information or data. With suitable communication devices, the message signals can be interchanged. Often the connection between two communication devices is not set up directly but rather via an intermediate station assigned to a plurality of communication devices. Said intermediate station serves for amplifying the carrier signals modulated by the message signals and for switching purposes if specific subscribers from a plurality of subscribers of a communication network are intended to be selected for the purpose of setting up a connection.
Customary communication devices are, for example, mobile telephones, which are used in motor vehicles or else as hand-held devices. In mobile radio systems, agreements have been reached on specific system standards. The digital mobile radio standards include GSM (Global System for Mobile Communications) and DECT (Digital Enhanced Cordless Telephone).
The basic construction of a communication device for mobile radio is disclosed in the publication by Siemens Aktiengesellschaft “ICs for Communications, Product Overview 07.96”, where the block diagram of a hand-held mobile telephone is shown on page 13. Accordingly, such a device has a transmission path and a reception path.
In the transmission path, the message to be transferred is recorded by a microphone, amplified in a baseband module and converted into a digital message signal. The latter is filtered and coded before being subjected to digital modulation. The modulated message signal is then in baseband, where it is filtered again after digital-to-analog conversion and transferred to a transmitter module which is electrically connected to the baseband module. The transmitter module converts the modulated message signal into the radiofrequency with which it is emitted via an antenna.
In the reception path, the signal received via the antenna is demodulated in a reception module and split into in-phase component and quadrature component. These components are in turn transferred via electrical connections, to the baseband module, where they are decoded in the reception path, which is independent of the transmission path, after filtering and analog-to-digital conversion. The decoded signal is again converted into an analog signal and amplified before being fed to a loudspeaker.
As has already been mentioned, the baseband module is electrically connected to the transmitter module and the reception module. Corresponding connections have to be provided in each case for the transmission path and reception path.